Add 'plus' key support

[KR155E]

What it does

Despite being mentioned in the keymaps package's readme, the "plus" key is unavailable for key bindings. This PR adds the necessary const to support it on a German QWERTZ keyboard.

How to test

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

@KR155E Can you add a How to test section? I've tried binding a command to Alt+Plus, but that didn't seem to work (I'm on QWERTZ).

[tsmaeder]

@KR155E ping?

[KR155E]

I am sorry, I have been unable to build Theia for a while and don't have the time to dive deeper into the problem. What I did was to hack the key constant into the bundled Theia (1.57.0) in my downstream IDE, then backport that constant. It did work in my working environment when I tried assigning simply "plus" as the keybinding for a custom command.